In gas-assisted injection molding, a pressurized gas is injected into a molten thermoplastic stream. One of the problems associated with such a system is back flow of the molten thermoplastic into the pressurized gas supply lines. The thermoplastic can harden in the pressurized gas supply lines thereby restricting gas flow and, ultimately, blocking gas flow. Several methods or approaches have been advanced to overcome this problem.
For example, in U.S. Pat. No. 4,905,901, a complicated baffle design was incorporated into the gas flow path to retard the flow of plastic into the gas flow path. The baffle was intended to provide a small diameter, tortuous passage through which the gas could easily flow but through which the viscous molten thermoplastic could flow only with great difficulty. And in U.S. Pat. No. 4,855,094, an attempt was made to solve this same problem by providing gas passages of sufficiently small diameter to resist entry of the molten thermoplastic. Gas passages of 0.005 to 0.040 inches were said to be effective to preventing entry of the molten thermoplastic material. Both of these approaches have been less successful than desired. Although such approaches may, in fact, restrict the entry of the thermoplastic into the gas passageways, they do not prevent such entry. Over time, the gas passages still become restricted and, ultimately, blocked. In some instances, it is necessary to clean out the gas passages every few hours.
Check valves with ball-shaped valve members have also been used in gas-assisted injection molding applications. For example, in U.S. Pat. No. 4,942,006, a ball check valve is provided in an injection nozzle. The actual valve is located within the gas passageway. When the valve is in the closed position, molten plastic entering the gas passageway can contact and coat at least a portion of the ball member. As the ball valve repeatedly moves from its open to closed positions, the ball may rotate. Portions of the ball which have contacted plastic material in earlier cycles may, in later cycles, act as the sealing surface. In such cases, plastic material on the sealing surfaces will prevent the valve from sealing. This failure to seal will allow even more plastic material to enter the gas passageway and contact the ball in later injection cycles, thus accelerating the problem. Ball valves of the type used in the prior art have, therefore, a limited useful lifetime and require frequent cleaning. In some instances, it may be necessary to clean such check valves every few operational hours. Such cleaning requires dismantling of the nozzle itself, which, depending on the nozzle design, can be very involved and time consuming.
It is, therefore, desirable to provide a method by which plastic material can be prevented, or at least minimized, from entering gas passageways in gas-assisted injection molding. The check valves of the present invention provide significantly improved performance over the methods currently in use to prevent plastic material from entering and clogging gas passageways in a gas-assisted injection molding apparatus. This improvement is accomplished by providing a check valve wherein the sealing surfaces are not in direct contact with the plastic material. When cleaning is necessary, the check valves of the present invention can be disassembled and cleaned quite easily. These check valves can also be used in other applications and environments.